There are a large number of prior art heating units which relate to making the heating distribution in high frequency heating units uniform. They are largely classified into a stirrer system in which metal vanes are turned in a heating chamber, a turntable system in which the object to be heated is turned and a rotary antenna system in which the antenna, which is the source of radiation of electromagnetic waves, is turned. Among them, the rotary antenna system which has small dimensions and which gives high uniformity of wave distribution is often utilized. The method of radiating electromagnetic waves from the bottom of the heating chamber using the rotary antenna system results in less nonuniform heating due to the standing waves inside the heating chamber, because the electromagnetic waves radiated are directly absorbed by the load, and therefore there is less influence from the dimensions of the heating chamber, which is an advantage, but it is defective in that the center of gyration is heated very intensively. As one of means for solving such a problem, there has been proposed a method comprising adjusting the length of the horizontal part of the rotary strip antenna, as reported in Japanese Laid-Open Patent application No. 15594 of 1981. According to this method, the overheating at the center of gyration is inhibited by adjusting the alignment of impedance between the horizontal rotary strip antenna and the object being heated. Therefore, if the shape and/or size of the load is changed, the radiation from the rotary strip antenna will be altered. Thus this method makes heating uniform for some limited loads, but has only a small effect on different loads.
For whatever load, it seems difficult with a strip antenna to diminish the radiation of electromagnetic waves at the center of gyration and propagate them in the horizontal direction.
As a method of propagating electromagnetic waves from the center of gyration in the horizontal direction, an arrangement for turning a flume shape rotary wave guide has been proposed, as disclosed in Japanese Patent Publication No. 2144 of 1973. In this arrangement, the coupling of the feeding port with the rotary waveguide is difficult. That is to say, because the direction of the electric field at the feeding port is fixed, when the rotary wave guide and the direction of the electric field coincide with each other, the electric wave is propagated through the flume shape rotary wave guide, but when they cross each other at a right angle, the electric waves are barely propagated. Thus in whichever direction the rotary waveguide is turned, the electric waves will in no event be propagated through the rotary wave guide. Accordingly, the heating distribution is differentiated between fore-and-aft and right-and-left.
In Japanese the arrangement disclosed in Utility Model Publication No. 35741 of 1972, with the antenna and the wave guide coupled, the rate of propagation of electric waves through the waveguide is unaltered, even if the turning direction is changed, but since the antenna and the wave guide are not electrically in contact with each other, not all of the electric waves on the antenna are propagated to the waveguide. On this account, it becomes necessary to provide for a labyrinth for the electric waves on the outer circumference of the waveguide, resulting in a complex waveguide.
In addition, a method of turning a waveguide having a plurality of openings with different radii of gyration at the bottom of an oven as disclosed in U.S. Pat. No. 4,314,127 has been contemplated. By this method, parts of the object being heated (food) near the openings are well heated, but its upper parts are only slightly heated like on a frying pan. Since it is impossible to equalize the rates of radiation of electric waves from the plurality of openings in accordance with whatever load is present such as various foods, consequently their distribution on a plane is not favorable.